Lymphoid tumors are the most frequent malignancies in children. Aberrant chromosomal rearrangements have been implicated in the generation of many of these malignancies. The long term goals of this subproject are to understand the mechanisms underlying genomic plasticity and its control in developing lymphocytes. The genes encoding lymphocyte antigen receptors are assembled by V(D)J recombination, a form of site-specific DNA rearrangement. V(D)J recombination is initiated by RAG-1 and RAG-2. These proteins form a protein complex that pairs and cleaves participating DNA segments at recombination signal sequences. Completion of recombination requires participation of several proteins that function in DNA double-strand break repair (DSBR). VB(D)J recombination is regulated in the cell cycle through by a mechanism in which RAG-2 protein is periodically degraded at the G1-S boundary. Accumulation of RAG-2 is specifically opposed by cyclin A/CDK2 and stimulated by the CDK inhibitor p27/Kip1, which coordinately induces G1 delay and V(D)J recombination. Enforced products that resemble those formed in the absence of non-homologous end joining (NHEJ), a type of DSBR essential for normal V(D)J rearrangement. Taken together, these observations suggest that the cell cycle regulation of RAG2-2 expression serves to coordinate initiation of V(D)J recombination with NHEJ. Renewed funding is requested for continued study of the regulation of RAG activity and its contribution to lymphoid tumorigenesis, with the following aims: (1) to define how RAG-1 and RAG-2 collaborate in the formation of synaptic complexes and DNA cleavage in vitro; (2) to test whether cell cycle regulation of RAG-2 expression serves to coordinate cleavage of substrate DNA with non-homologous end joining; (3) to determine the mechanism by which RAG-2 is periodically degraded in the cell cycle; and (4) to define the contribution of RAG activity to tumorigenesis in a mouse model for lymphocytic leukemia.